The lanthanide group of the rare earths, i.e., those having an atomic number of 57 through 71, have recently attracted attention because of their interesting optical, electrical and magnetic properties. For example, their potential technical application in communications and in data processing machines is well recognized. Depending on the chemical composition of various alloys of the lanthanides, their electrical conductivity can be modified over a wide range. More importantly, these materials are known to undergo dramatic changes in optical properties when subjected to changes in temperature, pressure or magnetic field. Possible applications of such "optical switching" materials range from dense optical information storage devices to switchable filters and components of integrated optical circuitry. The rare earth chalcogenides having these switching properties are denoted as mixed valence, mixed configuration, or fluctuating valence compounds.
The lanthanide metals are typically strongly reactive. Accordingly, the production of their pure tellurides, selenides or sulfides presents certain difficulties. The reaction exotherm of the metal/chalcogen is usually quite violent. Further, contamination with other materials particularly with oxygen is difficult to avoid.
Vapor deposition has played an important role in the preparation of thin film chalcogenide rare earths. Sputtering techniques have found some utility but, when rare earth metals are sputtered into a hydrogen sulfide atmosphere for example, the materials deposited are powders that are nonadherent and chemically unstable, reacting with oxygen readily. Alternatively, when thin film rare earth metals are deposited on an inert substrate and then exposed to deoxygenated hydrogen sulfide at high temperatures, the resulting films are typically contaminated with oxygen and contain little or no rare earth sulfide.
The reactive evaporation of a rare earth in a hydrogen sulfide atmosphere at a low partial pressure followed by deposition on a substrate has also yielded thin film material but the chalcogenide formed is not pure, being a mixture of mono, di and tri chalcogenides.
It is thus an object of this invention to present a method for producing chemically pure chalcogenides of the rare earths. With this method, these compounds can be produced directly in the form of thin films which are particularly useful for a variety of technical applications.
The method for producing thin films of chalcogenides of the rare earths is characterized by introducing a rare earth metal vapor into an atmosphere not containing oxygen, but containing gaseous chalcogens as well as hydrogen, at a total pressure about 1.times.10.sup.-4 to 9.times.10.sup.-4, giving rise to a reaction which forms gaseous rare earth chalcogenides. The gaseous rare earth chalcogenide is then deposited as a thin film on a substrate heated to 200.degree. C. to about 400.degree. C.